1998 Other Vauxhall Models Vauxhall omega 2.0l 16v fan belt

Engine Mechanical problem
1998 Other Vauxhall Models 4 cyl Two Wheel Drive 90000 miles

I've just done a fanbelt and tensioner on a vauxhall omega 2.0l 16v now every time i start it up the belt slips ove 1 of the pulley it doesnt fall all of the way of it just slips over 1 rib.

i was wondering if you had a diagram of how it went and if you could confirm the only reason that belt is slipping of is because one of the pulleys is not running true.

Thanks